High concentration, aqueous surfactant solutions are often desired or required for incorporation into a formulation or for conserving storage space or for economical transportation costs. In particular, aqueous liquid amphoteric and liquid anionic surfactant compositions having a total solids content greater than about 25%, preferably greater than about 35%, are desired, which can remain free-flowing and pumpable liquids at ambient room temperature.
One well known problem is that aqueous surfactant solutions, especially amphoteric and anionic surfactants, tend to be non-liquid, that is they either gel or become non-flowable and too viscous to pump at ambient room temperature, when the total solids content exceeds about 35%. Some prior attempts have been made to overcome this problem by adding fluidizers to the reaction mixture, for example, either during or after formation of the surfactant in an aqueous medium, or by introducing special additives or solvents. While some measure of success has been achieved, uneconomically large amounts of special additives, such as polyols and nonionic surfactants, may be required, and some fluidizers, such as solvents, are difficult or costly to remove from the medium. Some fluidizers may also introduce reaction byproducts and components that can interact with other ingredients in the formulation or interfere with the purpose for which the surfactant is subsequently used. Additionally, some fluidizers may introduce unwanted, toxicologically, physiologically, or environmentally unacceptable material.
In recent years, amphoteric surfactants, and in particular, betaine surfactants, have become increasingly desirable as ingredients in personal care products, such as cosmetics, toiletries, cosmeceuticals, topical pharmaceuticals, and the like, especially in products for cleansing the hair and skin. Consequently, there is an ongoing need for high concentration, aqueous surfactant compositions that remain flowable, pumpable liquids at ambient room temperature and contain minimal or no extraneous material from the reaction process. It has been surprisingly found that such a stable, flowable, pumpable, high concentration, aqueous, liquid surfactant composition can be prepared by the method disclosed herein.